INDUCTION OF APOPTOSIS BY PARTICULATE MATTER - ROLE OF TNF-ALPHA AND MAPK

Particulate matter (PM) is a major by-product from the combustion of f ossil fuels. The biological target of inhaled PM is the pulmonary epit helium and resident macrophages. In this study, we demonstrate that cu ltured macrophages (RAW 264.7 cells) exposed continuously to a well-de fined model of PM [benzo[a]pyrene adsorbed on carbon black (CB+BaP)] e xhibit a time-dependent expression and release of the cytokine tumor n ecrosis factor-alpha (TNF-alpha). CB+BaP also evoked programmed cell d eath or apoptosis in cultured macrophages as assessed by genomic DNA-l addering assays. The CB+BaP-induced apoptosis was inhibited when macro phages were treated with CB+BaP in the presence of a neutralizing anti body to TNF-alpha, suggesting that TNF-alpha plays an important role i n mediating CB+BaP-induced apoptosis in macrophages. Interestingly, ne ither untreated carbon black nor benzo[a]pyrene alone induced apoptosi s or caused the release of TNF-alpha in RAW 264.7 cells. Moreover, we observed that TNF-alpha. activates mitogen-activated protein kinase (M APK) activity, the extracellular signal-regulated kinases p42/p44, in a time-dependent manner. RAW 264.7 cells treated with PD-098059, a sel ective inhibitor of MAPK kinase activity, did not exhibit CB+BaP-induc ed apoptosis and TNF-alpha secretion. Furthermore, cells treated with the MAPK kinase inhibitor did not undergo TNF-alpha-induced apoptosis. Taken together, our data suggest that TNF-alpha mediates PM-induced a poptosis and that the MAPK pathway may play an important role in regul ating this pathway.